Add webgl2 suppression for ext-color-buffer-half-float.html

Already suppressed for webgl1, just needed for webgl2 to fix our FYI
testing.

TBR: shrekshao@google.com
Bug: angleproject:5038
Change-Id: I1bb5a6005ecfc09df13150f0f8082834675c5384
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2459390Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Shrek Shao <shrekshao@google.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814947}

content/test/gpu/gpu_tests/test_expectations/webgl2_conformance_expectations.txt

@@ -86,6 +86,7 @@ crbug.com/1100470 conformance/textures/misc/exif-orientation.html [ Skip ]
 crbug.com/1082533 [ mac intel ] conformance/textures/misc/texture-copying-and-deletion.html [ Failure ]
 crbug.com/1082533 [ win vulkan passthrough ] conformance/textures/misc/texture-copying-and-deletion.html [ Failure ]
 crbug.com/1018028 [ win vulkan ] conformance/rendering/bind-framebuffer-flush-bug.html [ Failure ]
+crbug.com/angleproject/5038 conformance/extensions/ext-color-buffer-half-float.html [ Skip ]
 
 crbug.com/953120 conformance/programs/program-handling.html [ Failure ]